This invention relates to digital loop carrier transmission systems and, in particular, to communication among common equipment peripherals within a remote and/or distant terminal.
In digital loop carrier transmission systems, peripherals within a remote or distant terminal are designated in terms of their function, and each may comprise a single circuit pack, a portion of a circuit pack, or several circuit packs within an equipment shelf or in different equipment shelves. For example, in the Subscriber Loop Carrier (SLC.RTM.) 2000 System, a metallic distribution controller (MDC) is located within a remote terminal and combines the functions of a transmit-receive unit (TRU) and a bank controller unit (BCU) in order to multiplex/demultiplex data and to control the other components of the remote terminal. Other peripherals which are part of the common equipment of the remote terminal include the system timing unit (STU) which handles the timing of the system, the virtual tributary unit (VTU) which acts as an interface to the optical portion of the network, and the provisioning and display control unit (PDC) which drives the display panel. These peripherals must be able to communicate with each other for the system to operate.
Typical communications systems for local area networks utilize a STAR configuration where each peripheral is coupled to a hub which controls transmission between the peripherals. Each peripheral could have a dedicated slot in which to send its messages, and these messages would be broadcast to all the peripherals (see, e.g., N. Abramson, "The Throughput of Packet Broadcasting Channels," IEEE Transactions on Communications, Vol. COM-25, No. 1 (Jan. 77), pp. 117-128). Such an approach was inefficient for a SLC.RTM. carrier system since a single central processing unit (CPU) at each peripheral would have to continually monitor incoming messages to see which messages were meant for that peripheral. Further, in many prior art STAR systems, it was necessary to detect collisions among messages from different peripherals and retransmit any messages that collided (see, e.g. R. M. Metcalfe et al., "Ethernet: Distributed Packet Switching for Local Computer Networks," Communications of the ACM, Vol. 19, No. 7 (July 1976), pp. 395-404 and U.S. Pat. No. 4,347,498 issued to Lee et al.). This requirement made the system unduly complex.
Other STAR network proposals have included the use of an alternate hub to take over communications in the event of a failure in the main hub (see, e.g., U.S. Pat. No. 4,701,756 issued to Burr).